Sample and hold circuitry is used in signal processing applications such as analog-to-digital converters. One type of sample and hold circuit includes a sampling switch, which is implemented using at least one transistor, and a capacitor. More particularly, a time-varying (i.e., analog) input signal being sampled is periodically switched to the capacitor during a sampling interval to store a charge on the capacitor, which represents a magnitude (e.g., a voltage level) of the input signal being sampled. Between each of the sampling intervals is a hold interval during which the voltage level stored on the capacitor is provided to an input of an analog-to-digital converter (ADC) that generates an n-bit binary number proportional to the stored voltage level, thereby representing an approximation of the input signal at the time of the sample. A limitation of the sampling switch is that the transconductance of the transistor used to create the switch varies based on the magnitude of the input signal. This is because the difference between input and control terminals of the sampling switch transistor (e.g., Vgs, which is the voltage difference between the gate and source terminals of a field effect transistor) varies with the magnitude of the input signal. These changes in Vgs introduce changes in the sampling switch transconductance causing distortion in the sampled signal.
Techniques, such as the use of a boosting circuit, reduce or eliminate changes in the transconductance of the sampling switch by maintaining a constant Vgs across the sampling switch; this keeps the impedance across the sampling switch constant to eliminate distortion. A primary limitation of boosting circuits, however, is the signal strength needed to not only drive the input to the sampling switch but also to drive the boosting circuit. Buffer circuits introduced to drive the boosting circuits cause the boosting circuits to maintain a constant Vgs only for input signals that are within V_sat of a supply voltage rail (e.g., Vdd or Vss), in oversampled systems, wherein V_sat represents overhead of an amplifier within the buffer circuit, which could be several hundred milivolts. Transistor devices used to implement the amplifier require some amount of bias voltage across the terminals to sustain proper operation; this bias voltage creates the amplifier overhead.
The present invention is illustrated by way of example, and it's not limited by the accompanying figures, in which like references indicate similar elements. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.